Air cavity packages typically include one or more semiconductor die attached to a base plate and an insulative window frame surrounding the die. A lid is placed over the window frame, sealing the die in a cavity of air. Air cavity packages are widely used to house high frequency devices such as radio-frequency or RF die. Packaging a high frequency semiconductor chip in an envelope of encapsulated air improves the high frequency properties of the die and the corresponding electrical leads, as opposed to being encapsulated in a molding compound which has a higher dielectric constant than air.
Some conventional air cavity packages use window frames made of ceramic, but the ceramic window frame is typically attached prior to the die attach process. Ceramic window frames are typically attached to a conductive metal base plate using a high temperature brazing process (e.g. 850 C). Such high processing temperatures preclude the use of copper base plates with ceramic window frames because the CTE (coefficient of thermal expansion) mismatch between copper and ceramic materials induces cracks in the ceramic window frame at brazing temperatures. As such, the base plate is typically made of CuMOCu or CuW when a ceramic window frame is used. However, both CuMOCu (260 W/mK) and CuW (180 W/mK) have a significantly lower thermal conductivity than pure copper (394 W/mK), reducing the overall thermal performance of the package.
Epoxies and other pliant materials can be used to attach a ceramic window frame to a metal base plate prior to die attach. However, pliant materials such as epoxies can be damaged during subsequent high temperature die attach processes. Some conventional air cavity assembly techniques involve attaching the window frame to the base plate after die attach, but the window frame is made of plastic, e.g. as disclosed in U.S. Pat. No. 6,511,866. A window frame made of plastic has a much lower thermal conductivity than a ceramic window frame, limiting the use of air cavity packages having plastic window frames to lower power applications where heat dissipation is less of a concern. Also, the materials and process which can be used for attaching a plastic window frame to a base plate are limited because of the low melting point of conventional plastic window frames.